U by Uniworld, a new river cruise brand originally restricted to passengers aged 18 to 40—then 21 to 45—has now dropped upper age limits altogether. When the brand's pair of sleek, all-black ships, dubbed simply the A and the B, launch in April, adults of any age can be on board, even if they have memories of watching Nixon's resignation or the last episode of M*A*S*H. In announcing the change,...